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diff --git a/python.html.markdown b/python.html.markdown deleted file mode 100644 index 0cc33a80..00000000 --- a/python.html.markdown +++ /dev/null @@ -1,827 +0,0 @@ ---- -language: python -contributors: - - ["Louie Dinh", "http://ldinh.ca"] - - ["Amin Bandali", "https://aminb.org"] - - ["Andre Polykanine", "https://github.com/Oire"] - - ["evuez", "http://github.com/evuez"] - - ["asyne", "https://github.com/justblah"] - - ["habi", "http://github.com/habi"] - - ["Rommel Martinez", "https://ebzzry.io"] -filename: learnpython.py ---- - -Python was created by Guido Van Rossum in the early 90s. It is now one of the -most popular languages in existence. I fell in love with Python for its -syntactic clarity. It's basically executable pseudocode. - -Feedback would be highly appreciated! You can reach me at [@louiedinh](http://twitter.com/louiedinh) -or louiedinh [at] [google's email service] - -Note: This article applies to Python 2.7 specifically, but should be applicable -to Python 2.x. Python 2.7 is reaching end of life and will stop being -maintained in 2020, it is though recommended to start learning Python with -Python 3. For Python 3.x, take a look at the [Python 3 tutorial](http://learnxinyminutes.com/docs/python3/). - -It is also possible to write Python code which is compatible with Python 2.7 -and 3.x at the same time, using Python [`__future__` imports](https://docs.python.org/2/library/__future__.html). `__future__` imports -allow you to write Python 3 code that will run on Python 2, so check out the -Python 3 tutorial. - -```python - -# Single line comments start with a number symbol. - -""" Multiline strings can be written - using three "s, and are often used - as comments -""" - -#################################################### -# 1. Primitive Datatypes and Operators -#################################################### - -# You have numbers -3 # => 3 - -# Math is what you would expect -1 + 1 # => 2 -8 - 1 # => 7 -10 * 2 # => 20 -35 / 5 # => 7 - -# Division is a bit tricky. It is integer division and floors the results -# automatically. -5 / 2 # => 2 - -# To fix division we need to learn about floats. -2.0 # This is a float -11.0 / 4.0 # => 2.75 ahhh...much better - -# Result of integer division truncated down both for positive and negative. -5 // 3 # => 1 -5.0 // 3.0 # => 1.0 # works on floats too --5 // 3 # => -2 --5.0 // 3.0 # => -2.0 - -# Note that we can also import division module(Section 6 Modules) -# to carry out normal division with just one '/'. -from __future__ import division - -11 / 4 # => 2.75 ...normal division -11 // 4 # => 2 ...floored division - -# Modulo operation -7 % 3 # => 1 - -# Exponentiation (x to the yth power) -2 ** 4 # => 16 - -# Enforce precedence with parentheses -(1 + 3) * 2 # => 8 - -# Boolean Operators -# Note "and" and "or" are case-sensitive -True and False # => False -False or True # => True - -# Note using Bool operators with ints -0 and 2 # => 0 --5 or 0 # => -5 -0 == False # => True -2 == True # => False -1 == True # => True - -# negate with not -not True # => False -not False # => True - -# Equality is == -1 == 1 # => True -2 == 1 # => False - -# Inequality is != -1 != 1 # => False -2 != 1 # => True - -# More comparisons -1 < 10 # => True -1 > 10 # => False -2 <= 2 # => True -2 >= 2 # => True - -# Comparisons can be chained! -1 < 2 < 3 # => True -2 < 3 < 2 # => False - -# Strings are created with " or ' -"This is a string." -'This is also a string.' - -# Strings can be added too! -"Hello " + "world!" # => "Hello world!" -# Strings can be added without using '+' -"Hello " "world!" # => "Hello world!" - -# ... or multiplied -"Hello" * 3 # => "HelloHelloHello" - -# A string can be treated like a list of characters -"This is a string"[0] # => 'T' - -# You can find the length of a string -len("This is a string") # => 16 - -# String formatting with % -# Even though the % string operator will be deprecated on Python 3.1 and removed -# later at some time, it may still be good to know how it works. -x = 'apple' -y = 'lemon' -z = "The items in the basket are %s and %s" % (x, y) - -# A newer way to format strings is the format method. -# This method is the preferred way -"{} is a {}".format("This", "placeholder") -"{0} can be {1}".format("strings", "formatted") -# You can use keywords if you don't want to count. -"{name} wants to eat {food}".format(name="Bob", food="lasagna") - -# None is an object -None # => None - -# Don't use the equality "==" symbol to compare objects to None -# Use "is" instead -"etc" is None # => False -None is None # => True - -# The 'is' operator tests for object identity. This isn't -# very useful when dealing with primitive values, but is -# very useful when dealing with objects. - -# Any object can be used in a Boolean context. -# The following values are considered falsey: -# - None -# - zero of any numeric type (e.g., 0, 0L, 0.0, 0j) -# - empty sequences (e.g., '', (), []) -# - empty containers (e.g., {}, set()) -# - instances of user-defined classes meeting certain conditions -# see: https://docs.python.org/2/reference/datamodel.html#object.__nonzero__ -# -# All other values are truthy (using the bool() function on them returns True). -bool(0) # => False -bool("") # => False - - -#################################################### -# 2. Variables and Collections -#################################################### - -# Python has a print statement -print "I'm Python. Nice to meet you!" # => I'm Python. Nice to meet you! - -# Simple way to get input data from console -input_string_var = raw_input( - "Enter some data: ") # Returns the data as a string -input_var = input("Enter some data: ") # Evaluates the data as python code -# Warning: Caution is recommended for input() method usage -# Note: In python 3, input() is deprecated and raw_input() is renamed to input() - -# No need to declare variables before assigning to them. -some_var = 5 # Convention is to use lower_case_with_underscores -some_var # => 5 - -# Accessing a previously unassigned variable is an exception. -# See Control Flow to learn more about exception handling. -some_other_var # Raises a name error - -# if can be used as an expression -# Equivalent of C's '?:' ternary operator -"yahoo!" if 3 > 2 else 2 # => "yahoo!" - -# Lists store sequences -li = [] -# You can start with a prefilled list -other_li = [4, 5, 6] - -# Add stuff to the end of a list with append -li.append(1) # li is now [1] -li.append(2) # li is now [1, 2] -li.append(4) # li is now [1, 2, 4] -li.append(3) # li is now [1, 2, 4, 3] -# Remove from the end with pop -li.pop() # => 3 and li is now [1, 2, 4] -# Let's put it back -li.append(3) # li is now [1, 2, 4, 3] again. - -# Access a list like you would any array -li[0] # => 1 -# Assign new values to indexes that have already been initialized with = -li[0] = 42 -li[0] # => 42 -li[0] = 1 # Note: setting it back to the original value -# Look at the last element -li[-1] # => 3 - -# Looking out of bounds is an IndexError -li[4] # Raises an IndexError - -# You can look at ranges with slice syntax. -# (It's a closed/open range for you mathy types.) -li[1:3] # => [2, 4] -# Omit the beginning -li[2:] # => [4, 3] -# Omit the end -li[:3] # => [1, 2, 4] -# Select every second entry -li[::2] # =>[1, 4] -# Reverse a copy of the list -li[::-1] # => [3, 4, 2, 1] -# Use any combination of these to make advanced slices -# li[start:end:step] - -# Remove arbitrary elements from a list with "del" -del li[2] # li is now [1, 2, 3] - -# You can add lists -li + other_li # => [1, 2, 3, 4, 5, 6] -# Note: values for li and for other_li are not modified. - -# Concatenate lists with "extend()" -li.extend(other_li) # Now li is [1, 2, 3, 4, 5, 6] - -# Remove first occurrence of a value -li.remove(2) # li is now [1, 3, 4, 5, 6] -li.remove(2) # Raises a ValueError as 2 is not in the list - -# Insert an element at a specific index -li.insert(1, 2) # li is now [1, 2, 3, 4, 5, 6] again - -# Get the index of the first item found -li.index(2) # => 1 -li.index(7) # Raises a ValueError as 7 is not in the list - -# Check for existence in a list with "in" -1 in li # => True - -# Examine the length with "len()" -len(li) # => 6 - -# Tuples are like lists but are immutable. -tup = (1, 2, 3) -tup[0] # => 1 -tup[0] = 3 # Raises a TypeError - -# You can do all those list thingies on tuples too -len(tup) # => 3 -tup + (4, 5, 6) # => (1, 2, 3, 4, 5, 6) -tup[:2] # => (1, 2) -2 in tup # => True - -# You can unpack tuples (or lists) into variables -a, b, c = (1, 2, 3) # a is now 1, b is now 2 and c is now 3 -d, e, f = 4, 5, 6 # you can leave out the parentheses -# Tuples are created by default if you leave out the parentheses -g = 4, 5, 6 # => (4, 5, 6) -# Now look how easy it is to swap two values -e, d = d, e # d is now 5 and e is now 4 - -# Dictionaries store mappings -empty_dict = {} -# Here is a prefilled dictionary -filled_dict = {"one": 1, "two": 2, "three": 3} - -# Look up values with [] -filled_dict["one"] # => 1 - -# Get all keys as a list with "keys()" -filled_dict.keys() # => ["three", "two", "one"] -# Note - Dictionary key ordering is not guaranteed. -# Your results might not match this exactly. - -# Get all values as a list with "values()" -filled_dict.values() # => [3, 2, 1] -# Note - Same as above regarding key ordering. - -# Get all key-value pairs as a list of tuples with "items()" -filled_dict.items() # => [("one", 1), ("two", 2), ("three", 3)] - -# Check for existence of keys in a dictionary with "in" -"one" in filled_dict # => True -1 in filled_dict # => False - -# Looking up a non-existing key is a KeyError -filled_dict["four"] # KeyError - -# Use "get()" method to avoid the KeyError -filled_dict.get("one") # => 1 -filled_dict.get("four") # => None -# The get method supports a default argument when the value is missing -filled_dict.get("one", 4) # => 1 -filled_dict.get("four", 4) # => 4 -# note that filled_dict.get("four") is still => None -# (get doesn't set the value in the dictionary) - -# set the value of a key with a syntax similar to lists -filled_dict["four"] = 4 # now, filled_dict["four"] => 4 - -# "setdefault()" inserts into a dictionary only if the given key isn't present -filled_dict.setdefault("five", 5) # filled_dict["five"] is set to 5 -filled_dict.setdefault("five", 6) # filled_dict["five"] is still 5 - -# You can declare sets (which are like unordered lists that cannot contain -# duplicate values) using the set object. -empty_set = set() -# Initialize a "set()" with a bunch of values -some_set = set([1, 2, 2, 3, 4]) # some_set is now set([1, 2, 3, 4]) - -# order is not guaranteed, even though it may sometimes look sorted -another_set = set([4, 3, 2, 2, 1]) # another_set is now set([1, 2, 3, 4]) - -# Since Python 2.7, {} can be used to declare a set -filled_set = {1, 2, 2, 3, 4} # => {1, 2, 3, 4} - -# Add more items to a set -filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5} - -# Do set intersection with & -other_set = {3, 4, 5, 6} -filled_set & other_set # => {3, 4, 5} - -# Do set union with | -filled_set | other_set # => {1, 2, 3, 4, 5, 6} - -# Do set difference with - -{1, 2, 3, 4} - {2, 3, 5} # => {1, 4} - -# Do set symmetric difference with ^ -{1, 2, 3, 4} ^ {2, 3, 5} # => {1, 4, 5} - -# Check if set on the left is a superset of set on the right -{1, 2} >= {1, 2, 3} # => False - -# Check if set on the left is a subset of set on the right -{1, 2} <= {1, 2, 3} # => True - -# Check for existence in a set with in -2 in filled_set # => True -10 in filled_set # => False -10 not in filled_set # => True - -# Check data type of variable -type(li) # => list -type(filled_dict) # => dict -type(5) # => int - - -#################################################### -# 3. Control Flow -#################################################### - -# Let's just make a variable -some_var = 5 - -# Here is an if statement. Indentation is significant in python! -# prints "some_var is smaller than 10" -if some_var > 10: - print "some_var is totally bigger than 10." -elif some_var < 10: # This elif clause is optional. - print "some_var is smaller than 10." -else: # This is optional too. - print "some_var is indeed 10." - -""" -For loops iterate over lists -prints: - dog is a mammal - cat is a mammal - mouse is a mammal -""" -for animal in ["dog", "cat", "mouse"]: - # You can use {0} to interpolate formatted strings. (See above.) - print "{0} is a mammal".format(animal) - -""" -"range(number)" returns a list of numbers -from zero to the given number -prints: - 0 - 1 - 2 - 3 -""" -for i in range(4): - print i - -""" -"range(lower, upper)" returns a list of numbers -from the lower number to the upper number -prints: - 4 - 5 - 6 - 7 -""" -for i in range(4, 8): - print i - -""" -While loops go until a condition is no longer met. -prints: - 0 - 1 - 2 - 3 -""" -x = 0 -while x < 4: - print x - x += 1 # Shorthand for x = x + 1 - -# Handle exceptions with a try/except block - -# Works on Python 2.6 and up: -try: - # Use "raise" to raise an error - raise IndexError("This is an index error") -except IndexError as e: - pass # Pass is just a no-op. Usually you would do recovery here. -except (TypeError, NameError): - pass # Multiple exceptions can be handled together, if required. -else: # Optional clause to the try/except block. Must follow all except blocks - print "All good!" # Runs only if the code in try raises no exceptions -finally: # Execute under all circumstances - print "We can clean up resources here" - -# Instead of try/finally to cleanup resources you can use a with statement -with open("myfile.txt") as f: - for line in f: - print line - - -#################################################### -# 4. Functions -#################################################### - -# Use "def" to create new functions -def add(x, y): - print "x is {0} and y is {1}".format(x, y) - return x + y # Return values with a return statement - - -# Calling functions with parameters -add(5, 6) # => prints out "x is 5 and y is 6" and returns 11 - -# Another way to call functions is with keyword arguments -add(y=6, x=5) # Keyword arguments can arrive in any order. - - -# You can define functions that take a variable number of -# positional args, which will be interpreted as a tuple by using * -def varargs(*args): - return args - - -varargs(1, 2, 3) # => (1, 2, 3) - - -# You can define functions that take a variable number of -# keyword args, as well, which will be interpreted as a dict by using ** -def keyword_args(**kwargs): - return kwargs - - -# Let's call it to see what happens -keyword_args(big="foot", loch="ness") # => {"big": "foot", "loch": "ness"} - - -# You can do both at once, if you like -def all_the_args(*args, **kwargs): - print args - print kwargs - - -""" -all_the_args(1, 2, a=3, b=4) prints: - (1, 2) - {"a": 3, "b": 4} -""" - -# When calling functions, you can do the opposite of args/kwargs! -# Use * to expand positional args and use ** to expand keyword args. -args = (1, 2, 3, 4) -kwargs = {"a": 3, "b": 4} -all_the_args(*args) # equivalent to all_the_args(1, 2, 3, 4) -all_the_args(**kwargs) # equivalent to all_the_args(a=3, b=4) -all_the_args(*args, **kwargs) # equivalent to all_the_args(1, 2, 3, 4, a=3, b=4) - - -# you can pass args and kwargs along to other functions that take args/kwargs -# by expanding them with * and ** respectively -def pass_all_the_args(*args, **kwargs): - all_the_args(*args, **kwargs) - print varargs(*args) - print keyword_args(**kwargs) - - -# Function Scope -x = 5 - - -def set_x(num): - # Local var x not the same as global variable x - x = num # => 43 - print x # => 43 - - -def set_global_x(num): - global x - print x # => 5 - x = num # global var x is now set to 6 - print x # => 6 - - -set_x(43) -set_global_x(6) - - -# Python has first class functions -def create_adder(x): - def adder(y): - return x + y - - return adder - - -add_10 = create_adder(10) -add_10(3) # => 13 - -# There are also anonymous functions -(lambda x: x > 2)(3) # => True -(lambda x, y: x ** 2 + y ** 2)(2, 1) # => 5 - -# There are built-in higher order functions -map(add_10, [1, 2, 3]) # => [11, 12, 13] -map(max, [1, 2, 3], [4, 2, 1]) # => [4, 2, 3] - -filter(lambda x: x > 5, [3, 4, 5, 6, 7]) # => [6, 7] - -# We can use list comprehensions for nice maps and filters -[add_10(i) for i in [1, 2, 3]] # => [11, 12, 13] -[x for x in [3, 4, 5, 6, 7] if x > 5] # => [6, 7] - -# You can construct set and dict comprehensions as well. -{x for x in 'abcddeef' if x in 'abc'} # => {'a', 'b', 'c'} -{x: x ** 2 for x in range(5)} # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16} - - -#################################################### -# 5. Classes -#################################################### - -# We subclass from object to get a class. -class Human(object): - # A class attribute. It is shared by all instances of this class - species = "H. sapiens" - - # Basic initializer, this is called when this class is instantiated. - # Note that the double leading and trailing underscores denote objects - # or attributes that are used by python but that live in user-controlled - # namespaces. You should not invent such names on your own. - def __init__(self, name): - # Assign the argument to the instance's name attribute - self.name = name - - # Initialize property - self.age = 0 - - # An instance method. All methods take "self" as the first argument - def say(self, msg): - return "{0}: {1}".format(self.name, msg) - - # A class method is shared among all instances - # They are called with the calling class as the first argument - @classmethod - def get_species(cls): - return cls.species - - # A static method is called without a class or instance reference - @staticmethod - def grunt(): - return "*grunt*" - - # A property is just like a getter. - # It turns the method age() into an read-only attribute - # of the same name. - @property - def age(self): - return self._age - - # This allows the property to be set - @age.setter - def age(self, age): - self._age = age - - # This allows the property to be deleted - @age.deleter - def age(self): - del self._age - - -# Instantiate a class -i = Human(name="Ian") -print i.say("hi") # prints out "Ian: hi" - -j = Human("Joel") -print j.say("hello") # prints out "Joel: hello" - -# Call our class method -i.get_species() # => "H. sapiens" - -# Change the shared attribute -Human.species = "H. neanderthalensis" -i.get_species() # => "H. neanderthalensis" -j.get_species() # => "H. neanderthalensis" - -# Call the static method -Human.grunt() # => "*grunt*" - -# Update the property -i.age = 42 - -# Get the property -i.age # => 42 - -# Delete the property -del i.age -i.age # => raises an AttributeError - -#################################################### -# 6. Modules -#################################################### - -# You can import modules -import math - -print math.sqrt(16) # => 4.0 - -# You can get specific functions from a module -from math import ceil, floor - -print ceil(3.7) # => 4.0 -print floor(3.7) # => 3.0 - -# You can import all functions from a module. -# Warning: this is not recommended -from math import * - -# You can shorten module names -import math as m - -math.sqrt(16) == m.sqrt(16) # => True -# you can also test that the functions are equivalent -from math import sqrt - -math.sqrt == m.sqrt == sqrt # => True - -# Python modules are just ordinary python files. You -# can write your own, and import them. The name of the -# module is the same as the name of the file. - -# You can find out which functions and attributes -# defines a module. -import math - -dir(math) - - -# If you have a Python script named math.py in the same -# folder as your current script, the file math.py will -# be loaded instead of the built-in Python module. -# This happens because the local folder has priority -# over Python's built-in libraries. - - -#################################################### -# 7. Advanced -#################################################### - -# Generators -# A generator "generates" values as they are requested instead of storing -# everything up front - -# The following method (*NOT* a generator) will double all values and store it -# in `double_arr`. For large size of iterables, that might get huge! -def double_numbers(iterable): - double_arr = [] - for i in iterable: - double_arr.append(i + i) - return double_arr - - -# Running the following would mean we'll double all values first and return all -# of them back to be checked by our condition -for value in double_numbers(range(1000000)): # `test_non_generator` - print value - if value > 5: - break - - -# We could instead use a generator to "generate" the doubled value as the item -# is being requested -def double_numbers_generator(iterable): - for i in iterable: - yield i + i - - -# Running the same code as before, but with a generator, now allows us to iterate -# over the values and doubling them one by one as they are being consumed by -# our logic. Hence as soon as we see a value > 5, we break out of the -# loop and don't need to double most of the values sent in (MUCH FASTER!) -for value in double_numbers_generator(xrange(1000000)): # `test_generator` - print value - if value > 5: - break - -# BTW: did you notice the use of `range` in `test_non_generator` and `xrange` in `test_generator`? -# Just as `double_numbers_generator` is the generator version of `double_numbers` -# We have `xrange` as the generator version of `range` -# `range` would return back and array with 1000000 values for us to use -# `xrange` would generate 1000000 values for us as we request / iterate over those items - -# Just as you can create a list comprehension, you can create generator -# comprehensions as well. -values = (-x for x in [1, 2, 3, 4, 5]) -for x in values: - print(x) # prints -1 -2 -3 -4 -5 to console/terminal - -# You can also cast a generator comprehension directly to a list. -values = (-x for x in [1, 2, 3, 4, 5]) -gen_to_list = list(values) -print(gen_to_list) # => [-1, -2, -3, -4, -5] - -# Decorators -# A decorator is a higher order function, which accepts and returns a function. -# Simple usage example – add_apples decorator will add 'Apple' element into -# fruits list returned by get_fruits target function. -def add_apples(func): - def get_fruits(): - fruits = func() - fruits.append('Apple') - return fruits - return get_fruits - -@add_apples -def get_fruits(): - return ['Banana', 'Mango', 'Orange'] - -# Prints out the list of fruits with 'Apple' element in it: -# Banana, Mango, Orange, Apple -print ', '.join(get_fruits()) - -# in this example beg wraps say -# Beg will call say. If say_please is True then it will change the returned -# message -from functools import wraps - - -def beg(target_function): - @wraps(target_function) - def wrapper(*args, **kwargs): - msg, say_please = target_function(*args, **kwargs) - if say_please: - return "{} {}".format(msg, "Please! I am poor :(") - return msg - - return wrapper - - -@beg -def say(say_please=False): - msg = "Can you buy me a beer?" - return msg, say_please - - -print say() # Can you buy me a beer? -print say(say_please=True) # Can you buy me a beer? Please! I am poor :( -``` - -## Ready For More? - -### Free Online - -* [Automate the Boring Stuff with Python](https://automatetheboringstuff.com) -* [Learn Python The Hard Way](http://learnpythonthehardway.org/book/) -* [Dive Into Python](http://www.diveintopython.net/) -* [The Official Docs](http://docs.python.org/2/) -* [Hitchhiker's Guide to Python](http://docs.python-guide.org/en/latest/) -* [Python Module of the Week](http://pymotw.com/2/) -* [A Crash Course in Python for Scientists](http://nbviewer.ipython.org/5920182) -* [First Steps With Python](https://realpython.com/learn/python-first-steps/) -* [LearnPython](http://www.learnpython.org/) -* [Fullstack Python](https://www.fullstackpython.com/) - -### Dead Tree - -* [Programming Python](http://www.amazon.com/gp/product/0596158106/ref=as_li_qf_sp_asin_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596158106&linkCode=as2&tag=homebits04-20) -* [Dive Into Python](http://www.amazon.com/gp/product/1441413022/ref=as_li_tf_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1441413022&linkCode=as2&tag=homebits04-20) -* [Python Essential Reference](http://www.amazon.com/gp/product/0672329786/ref=as_li_tf_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0672329786&linkCode=as2&tag=homebits04-20) |